Flints sometimes have a fossil at their core, which acted as a nucleus for the formation of the flint. Sometimes the whole fossil (and only the fossil) becomes flint; this happens with sea urchins, for instance.

I think what you have here is a result of a collection of shell fragments acting as nuclei for flint formation. And as the flints 'grew', they merged together. Now, the fossils are made of flint as well, so the lump is all the same hardness, which makes it weather/erode similarly, causing the lump to be fairly smooth.

But...

"There are no known sources of flint nodules in situ in Scotland, but several deposits of

flint gravels are well known. These are most abundant in the Buchan district of Aberdeenshire"

...

"In addition to these mapped sources flint is plentiful on many beaches"

Well done for the research.And to some extent you could be right. But there isn't really enough in the specimen to call it a septarian nodule. It is a piece of flint or chert with some contrastingly-coloured parts, which could be veins of septarian nature.

And please don't be misled by the text in your link:

"This Painting is an Septarian Nodule crystal,They were Formed 100 million years ago.When the Gulf of Mexicio reached what is now southern Utah.De-composing sea(shell)life,killed by Volcanic eruptuns."

They are not 'crystal', though the veins contain crystals, they were not formed at any particular time, are only vaguely related to 'sea(shell)life', and that life was not 'killed by Volcanic eruptuns'.